(1) Field of the Invention
The present invention relates to the use of molten salts for hazardous or toxic waste destruction. The invention further relates to the dispensing of molten materials through an aperture heated or cooled to open or seal the aperture.
(2) Description of the Related Art
In a molten salt reactor for the disposal of a hazardous material, the material is fed into a reaction vessel containing a molten salt, typically sodium carbonate at 900-1000xc2x0 C. In the salt, flameless oxidation converts organic components of the material into CO2, N2, and H2O which are removed as gases; acid-forming components of the material are converted into salts such as NaCl, which are retained in the molten salt; and the remaining components, such as metals, are also retained in the salt in metallic or oxide form. The vessel is provided with a suitable heater, typically electric, to melt the salt initially; and a heater may be required continually if reactions in the salt do not release sufficient heat to keep it molten. Some hazardous materials have sufficient oxygen for the oxidation; but if not, air is injected into the salt.
The molten salt is drained from time to time and allowed to solidify, thereby removing and capturing the materials retained in the molten salt. The solidified material is then processed to remove the retained material in ways appropriate thereto, and the salt is recycled to the reactor.
Since the molten salt is, typically, highly corrosive and erosive to most engineering materials, valves for such materials are impractical for the retention and draining of the molten salt. Therefore, it is known to provide the vessel with an exit conduit, which is normally at a temperature below the melting point of the salt, but which is provided with a drain heater to heat the conduit above this point. Initially, a plug of frozen salt is established in the conduit to retain the molten salt in the vessel during the disposal reaction, and the salt and retained materials are drained by energizing the drain heater to melt the plug.
Insofar as known to the applicants, heretofore in this xe2x80x9cfrozen plugxe2x80x9d approach for draining molten salt from a reactor for waste disposal, the frozen salt plug was formed by providing the exit end of the drain conduit with a mechanical plug and allowing molten salt from the vessel to enter the conduit and solidify at the mechanical plug, the existence of the frozen salt plug being confirmed after its formation by removing the mechanical plug. Also, the amount of salt required in the vessel to establish the salt plug was a substantial portion of the total amount required for the molten salt reaction; for example, 50 kg. of new or recycled salt was required to establish the plug for a total amount of 160 kg.
However, applications of a plug by freezing a molten material to close a conduit and then melting the plug to reopen the conduit are known for a variety of substances other than molten salts. In these applications, an additional mechanical block may or may not be used, and it is known to use and variously control a number of forms of energy for melting the plug. Representative examples of such applications are provided in the following four United States patents:
In U.S. Pat. No. 4,083,478, which issued 11 Apr. 1978 to McLane for a xe2x80x9cMolten metal shut-off valve utilizing cooling to solidify metal flowxe2x80x9d, a restricted passageway for molten lead is xe2x80x9csurrounded by a coolant chamber which, when supplied with coolant, cold water for example, chills the passageway very rapidly to solidify the lead flowing therethrough into a solid slug which then blocks the passageway against further flowxe2x80x9d, the passageway being located so xe2x80x9cthat upon draining of the coolant chamber the solidified slug in the restricted passageway is quickly heated to the melting temperature of the lead to re-establish the flow through the valve . . . . xe2x80x9d
U.S. Pat. No. 4,271,993, which issued 9 Jun. 1981 to Anderson for a xe2x80x9cSlag tap for coal slagging gasifierxe2x80x9d in which xe2x80x9ccontainment materials . . . are subject to aggressive erosion, corrosion and thermal attack by the [molten] slag and ironxe2x80x9d, discloses tapping thereof xe2x80x9cinitiated and controlled by stopping or reducingxe2x80x9d the output of a burner.
U.S. Pat. No. 5,649,992 issued 22 Jul. 1997 to Carter, Jr. for xe2x80x9cMethods for flow control in electroslag refining processxe2x80x9d discloses a variety of ways for controlling the flow of molten refined metal xe2x80x9cby coordinating, among other parameters: the rate of melting of the unrefined metal; the hydrostatic head of molten metal and slag above the bottom pour cold finger orifice; the rate of induction heat supplied to the metal within the cold finger apparatus; the rate of heat removal from the metal within the cold finger apparatus through the cold finger apparatus itself and through adjacent gas cooling means; and by applying electromagnetic force to selectively speed up, slow down and/or interrupt the flow of metal through the cold finger apparatus via an electromagnetic orifice, preferably utilizing a processor, such as a computer.xe2x80x9d
U.S. Pat. No. 6,210,629 issued 3 Apr. 2001 to Bruckner, et al. for a xe2x80x9cMethod and device for discontinuous parting off of molten massxe2x80x9d wherein an xe2x80x9cinductor surrounds a passage through which the melt is discharged from a vessel. The inductor applies radial electromagnetic energy to the passage to maintain the melt in a molten state. To stop flow of the melt through the passage, an outlet opening of the passage is closed and the inductor is electrically switched off while a cooling medium is flowed through the inductor. To begin flow of the melt, the outlet opening of the passage is opened and the inductor is electrically switched on.xe2x80x9d For redundancy in opening and closing, the passage is provided with a mechanical closure as well as the described device.
However, due to the properties of molten salt and systems using molten salt, such a plug and removal system has not been attempted. Also, due to these properties and the use of mechanical plugs and vacuuming means necessary to remove excess molten salt have made molten salt systems expensive and dangerous to operate. Therefore, for many uses, these systems have not been commercially employed. Thus, it is desired to develop a molten salt system plug and removal system and method that does not employ a mechanical plug and employs residue molten salt from within the system.
The present invention is concerned with apparatus and methods for selectively draining melt material such as molten salt material which has reacted in a vessel to dispose of waste materials, particularly energetic materials. The invention utilizes a meltable and reclosable drain plug disposed in a drain conduit otherwise unobstructed by mechanical closures such as valves or caps. The conduit is used with a first, or drain, heater at the vessel and a second, or plug forming, heater spaced from the vessel. These heaters may utilize any form of energy, electrical energization being convenient.
The conduit is detachably connected to a drain opening of the vessel in any suitable manner, as by screw threads, the conduit being detached from and reattached to the vessel when the first heater is de-energized and when the vessel does not contain molten melt material, as when the vessel is empty or the reaction material solidified.
Initially, the conduit is detached from the vessel; placed at the second heater; and partially filled with a predetermined quantity of plugging material which is in flowable form and is sufficient to block the conduit. This material is, preferably, the same salt material used for reaction in the vessel so as to not contaminate the drained material. Such salt material is, typically, provided in particulate form, and can be retained in the conduit by any suitable closure. The second heater is first energized to melt the plugging material and is then de-energized so that a solid plug of this material forms in the conduit.
The conduit, including the plug, is removed from the closure and the second heater and is then reattached to the vessel. The molten reaction material is then formed in the vessel by heat supplied by sources which are not directly involved in the present invention. The conduit is, typically, constructed, of material effective for heat transfer; but is disposed in relation to the vessel so that the plug does not melt because heat transfer to the conduit from the vessel is limited, and heat transfer to the environment keeps the temperature of the plug below the melting point of the plugging material.
When the melt material is to be drained, the first heater is energized to heat the conduit and melt the plug so that the melt material drains through the unobstructed conduit.
A new plug may be established in the conduit by removing the conduit from the vessel to the second heater, as above described. However, when the conduit is suitably configured, a quantity of the melt material corresponding to the above-specified predetermined quantity may remain in the conduit after draining the vessel. With this configuration, the first heater is de-energized after substantially draining the vessel so that the melt material in the conduit solidifies to form another plug in the conduit.
It is an object of the present invention to provide for draining process materials that corrode, erode, and jam mechanical closures.
Another object is to provide such draining by a meltable plug formed conveniently with a small quantity of plugging material that does not contaminate drained process material.
Yet another object is to provide for the formation of such a plug, after draining a reaction vessel, without cooling the vessel or application of mechanical closures at the vessel.
Further objects are to provide such draining with apparatus which is simple in construction and methods which is simple in operation, the apparatus and methods being dependable and being particularly and fully effective with molten salts used for waste disposal.